BACKGROUND AND PRIOR ART
It is well known that the phenomena of vision is assured by radiation solely in the visible spectrum, that is to say, electromagnetic radiation having a wave length between about 400 and 700 nm. This radiation is not dangerous to the eye if its intensity is not too great, in contrast with ultraviolet and infrared radiation which can produce severe lesions even at a relatively weak dosage. The ideal protection filter for the eye should therefore have a maximum factor of transmission in the domain of greatest sensitivity of the eye to attenuate the visible light in order to avoid brightness or glare in an intense luminous flux and to totally eliminate ultraviolet and infrared radiation by absorption or reflection.
The protection of the eyes against objectionable radiation in the electromagnetic spectrum is most often obtained by the use of semitransparent mineral or organic glass generally referred to as "sunglasses". The partial transmission in the visable light region is generally obtained by introducing a colorant into the body of the glass or plastic material. Certain compositions permit achieving a good protection of the eyes but necessitate a coating of special expensive materials. The lenses of sunglasses widely distributed in the market do not generally contain such materials and lead to very poor protection since they reduce the visible radiation but allow passage of a substantial portion of the ultraviolet and infrared radiation. The eye compensates for the reduction of the visible light by a greater opening of the iris and, under these conditions, receives a much greater amount of harmful energy than in the absence of any protection.
In addition to the great variety of tinted mineral and organic glasses, other solutions have been proposed for the protection of the eyes and notably the utilization of filters comprising a support of glass or plastic material covered with one or a plurality of thin metallic layers particularly of gold or copper. U.S. Pat. Nos. 3,118,781; 1,222,049; 2,087,802; and 2,854,349 concern such filters. Nevertheless, these filters which give satisfactory results in a great number of utilizations, for example, the protection of the eyes of welders, cannot be employed generally and cannot be applied to sunglasses, in particular because the internal surface of this type of filter is very reflective and the eye receives, without filtering, the greatest portion of the light arriving at the rear and reflected by the filter.
Another known solution consists of utilizing mixed filters composed of a support covered with a metallic layer isolated by two transparent layers. In addition to the problems posed by the practical realization of such filters, in particular the control of the thickness of the layers, the obtained spectrum is still remote from that of the ideal filter.
This type of filter has been perfected, particularly for the purpose of manufacturing of sunglasses, as shown in French Pat. No. 2003177 by covering the internal face of the filter with a metallic layer of much smaller thickness than the metallic filtering layer which serves as an anti-reflection layer which eliminates in part the reflection of light at the rear face of the filter. However, this filter retains the disadvantage of layers having metallic filters which considerably attenuate the visible range without providing sufficient protection in the ultraviolet and infrared ranges while additionally it does not assure sufficient protection in the case of intense ultraviolet and infrared emission.
FIG. 1 of the attached drawings allow a comparison of the transmission curves as a function of the wave length of an ideal filter (curve 1), conventional sunglasses (curve 2), and a mixed filter such as that described in French patent 2000177 (curve 3). As can be readily seen, none of the tested filters assures protection in the infrared region and in the mixed filter described in the French patent, the percentage of visible light which is transmitted is very low.